diff --git a/changelog/new_add_railsactioncontrollerflashbeforerender.md b/changelog/new_add_railsactioncontrollerflashbeforerender.md
new file mode 100644
index 0000000000..fd97953452
--- /dev/null
+++ b/changelog/new_add_railsactioncontrollerflashbeforerender.md
@@ -0,0 +1 @@
+* [#759](https://github.com/rubocop/rubocop-rails/pull/759): Add new `Rails/ActionControllerFlashBeforeRender` cop. ([@americodls][])
diff --git a/config/default.yml b/config/default.yml
index 284b45cb11..f69e5528a5 100644
--- a/config/default.yml
+++ b/config/default.yml
@@ -65,6 +65,16 @@ Rails:
   Enabled: true
   DocumentationBaseURL: https://docs.rubocop.org/rubocop-rails
 
+Rails/ActionControllerFlashBeforeRender:
+  Description: 'Use `flash.now` instead of `flash` before `render`.'
+  StyleGuide: 'https://rails.rubystyle.guide/#flash-before-render'
+  Reference: 'https://api.rubyonrails.org/classes/ActionController/FlashBeforeRender.html'
+  Enabled: 'pending'
+  SafeAutocorrect: true
+  VersionAdded: '2.15'
+  Include:
+    - '**/test/**/*.rb'
+
 Rails/ActionControllerTestCase:
   Description: 'Use `ActionDispatch::IntegrationTest` instead of `ActionController::TestCase`.'
   StyleGuide: 'https://rails.rubystyle.guide/#integration-testing'
diff --git a/lib/rubocop/cop/rails/action_controller_flash_before_render.rb b/lib/rubocop/cop/rails/action_controller_flash_before_render.rb
new file mode 100644
index 0000000000..632d65c28a
--- /dev/null
+++ b/lib/rubocop/cop/rails/action_controller_flash_before_render.rb
@@ -0,0 +1,48 @@
+# frozen_string_literal: true
+
+module RuboCop
+  module Cop
+    module Rails
+      # Using `flash` assignment before `render` will persist the message for too long.
+      # Check https://guides.rubyonrails.org/action_controller_overview.html#flash-now
+      #
+      # @example
+      #   # bad
+      #   flash[:alert] = "Warning!"
+      #   render :index
+      #
+      #   # good
+      #   flash.now[:alert] = "Warning!"
+      #   render :index
+      #
+      class ActionControllerFlashBeforeRender < Base
+        extend AutoCorrector
+
+        MSG = 'Use `flash.now` before `render`.'
+
+        def_node_search :flash_before_render?, <<~PATTERN
+          ^(send (send nil? :flash) :[]= ...)
+        PATTERN
+
+        def_node_search :render?, <<~PATTERN
+          (send nil? :render ...)
+        PATTERN
+
+        RESTRICT_ON_SEND = [:flash].freeze
+
+        def on_send(node)
+          expression = flash_before_render?(node)
+
+          return unless expression
+
+          context = node.parent.parent
+          return unless context.descendants.any? { render?(_1) }
+
+          add_offense(node) do |corrector|
+            corrector.replace(node, 'flash.now')
+          end
+        end
+      end
+    end
+  end
+end
diff --git a/lib/rubocop/cop/rails_cops.rb b/lib/rubocop/cop/rails_cops.rb
index 2f1e0996d1..c81b246b95 100644
--- a/lib/rubocop/cop/rails_cops.rb
+++ b/lib/rubocop/cop/rails_cops.rb
@@ -9,6 +9,7 @@
 require_relative 'mixin/target_rails_version'
 
 require_relative 'rails/action_controller_test_case'
+require_relative 'rails/action_controller_flash_before_render'
 require_relative 'rails/action_filter'
 require_relative 'rails/active_record_aliases'
 require_relative 'rails/active_record_callbacks_order'
diff --git a/spec/rubocop/cop/rails/action_controller_flash_before_render_spec.rb b/spec/rubocop/cop/rails/action_controller_flash_before_render_spec.rb
new file mode 100644
index 0000000000..3fb8998174
--- /dev/null
+++ b/spec/rubocop/cop/rails/action_controller_flash_before_render_spec.rb
@@ -0,0 +1,30 @@
+# frozen_string_literal: true
+
+RSpec.describe RuboCop::Cop::Rails::ActionControllerFlashBeforeRender, :config do
+  it 'registers an offense when using `flash` before `render`' do
+    expect_offense(<<~RUBY)
+      flash[:alert] = "msg"
+      ^^^^^ Use `flash.now` before `render`.
+      render :index
+    RUBY
+
+    expect_correction(<<~RUBY)
+      flash.now[:alert] = "msg"
+      render :index
+    RUBY
+  end
+
+  it 'does not register an offense when using `flash.now` before `render`' do
+    expect_no_offenses(<<~RUBY)
+      flash.now[:alert] = "msg"
+      render :index
+    RUBY
+  end
+
+  it 'does not register an offense when using `flash` before `redirect_to`' do
+    expect_no_offenses(<<~RUBY)
+      flash[:alert] = "msg"
+      redirect_to "https://www.rubygems.org"
+    RUBY
+  end
+end